This invention relates to a method of manufacturing a high refractive index (HRI) film and an apparatus and a system for accomplishing such a method.
Typically, high refractive index film refers to a film that is semi-transparent to the human eye. It may be transparent when viewed from one angle, but not from another angle. Included within the uses of this type of film are decorative wrapping paper, diffraction gratings and holograms. Holograms create the appearance of a three-dimensional image from a two dimensional object. Among the many uses of holograms are identification cards, trading cards, such as those picturing sports players, credit cards, and artistic uses.
Conventionally, high refractive index film is manufactured by depositing a layer of material onto a transparent plastic film. This may be accomplished by exposing the film in a vacuum chamber to a vapor which deposits on the film to create a layer. The deposited layer causes the refraction. This layer may be deposited as the film is fed from a feed spool to a take up spool. This method of creating a high refractive index film is generally described in U.S. Pat. No. 5,107,791 issued to Hirokawa et al. and U.S. Pat. No. 5,351,142 issued to Cueli.
The material heated to form the deposition layer varies. For example, Hirokawa teaches heating a composition composed mainly of a combination of silicon and silicon oxide or silicon oxide alone. However, zinc sulfide may also be used. Conventionally, this material is placed in a crucible or bowl like structure and heated through conduction. This is explained in detail in Hirokawa. A crucible like structure has several disadvantages. For instance, since the top of a crucible is generally open and the walls of the crucible are generally perpendicular to the top of the crucible, the vapor produced from heating the deposition material can diffuse out of the crucible in an uncontrolled fashion. More particularly, as the vapor is formed it diffuses upward and then exits the crucible. Upon exiting the crucible, the vapor can diffuse in almost any direction. Experience with crucibles shows that the vapor diffuses outward in addition to upward. Since the film is disposed above the crucible, it is preferable that the vapor diffuse upward and not outward so that the vapor contacts the film and deposits on it. Vapor that diffuses outward slows the deposition rate onto the film. This slows the rate at which film can be transferred from the feed roll to the take up roll and the overall rate of production of HRI film.
Another drawback of bowl shaped crucibles is that when they are used in conjunction with zinc sulfide, the zinc sulfide tends to form a crusty like layer across the top of the crucible when it is heated. This occurs because the zinc sulfide pellets or tablets at the top of the crucible form a layer and become supported by the zinc sulfide pellets beneath the layer. This layer limits the flow rate of vapor from the crucible. As described above, this then slows the feed rate of film and the rate of production.
Conventional crucibles also have a fixed opening at the top. Because of this, the flow rate of vapor produced by the crucible cannot be adjusted easily. Rather, assuming the same amount of heat and deposition material, a new crucible would have to be manufactured with a different size opening at the top in order to produce the same flow rate. This is inefficient and costly, particularly since the flow rate may have to be varied depending on the specific characteristics of the vacuum chamber and system employed.
In addition to bowl like crucibles, continuous vaporizers have also been designed. Hirokawa, for example, teaches a vaporizer that has an opening at either axial end through which deposition material can be continuously fed. At one end, the deposition material is inserted into the vaporizer. It is pushed through the vaporizer and is heated. As it is heated, vapor is produced and exhausts through a fixed opening in the top of the vaporizer. Such a system also has its disadvantages. For instance, the opening in the vaporizer taught by Hirokawa is fixed and cannot be adjusted to vary the flow rate of vapor. In light of the prior art, an improved method of heating a deposition material is needed that is simple, yet allows the flow rate of vapor to be varied by adjusting the vaporizer. Furthermore, an apparatus and a system for accomplishing such a method is needed.